PPPoE is the point-to-point protocol over Ethernet. It needs to talk to the actual ethernet hardware (i.e. data link layer) to work, not to a protocol stack in the network/transport layer.

http://en.wikipedia.org/wiki/OSI_model

This means, in plain terms, that the ethernet modem does not "talk TCP/IP" with the device that is using it - it "talks ethernet". Therefore, you cannot access the modems services if all you have is TCP/IP.